With development of wireless multimedia services, there is an increasing need for enhanced data rate and enhanced user experience are put forward, demanding more from capacity and coverage of a conventional cellular network system. On the other hand, popularity of applications, such as for social networking, short-distance data sharing, local advertising, etc., leads to an increasing need for knowledge of, and communication with, a nearby person or thing of interest (which may be referred to as a proximity service). A cell-based conventional cellular network can provide but limited support to an enhanced data rate as well as a proximity service. Birth of D2D technology, well in response to such needs, signals a replacement trend of future development of communication. Using D2D technology, traffic on a cellular network may be offloaded, battery power consumption at a UE may be reduced, data rate may be enhanced, and robustness of a network infrastructure may be improved, well meeting the needs for the enhanced data rate and the proximity service.
D2D technology may operate at an authorized or unauthorized frequency band, and allow multiple UE supporting D2D function (referred to as D2D UE) to perform direct discovery/direct communication with or without a network infrastructure. D2D communication applies mainly in three modes as follows.
1) In a mode 1 as shown in FIG. 1, a UE 1 and a UE 2 exchange data covered by a cellular network, where user-plane data go through no network infrastructure.
2) In a mode 2 as shown in FIG. 1, relayed transmission, for a UE4 with a poor signal in an area of weak or no coverage, allows the UE4 to communicate with a network via a nearby UE 3 covered by the network, which helps an operator in extending coverage and enhancing capacity.
3) In a mode 3 as shown in FIG. 1, direct inter-device communication is allowed, such as in case of a cellular network failure due to an earthquake or another emergency, where single-hop or multi-hop communication of both control-plane data and user plane data is performed among a UE5, a UE6 and a UE7, through no network infrastructure.
D2D technology generally involves D2D discovery and D2D communication. D2D discovery is for determining proximity of two or more D2D UE to each other (such as within a range of direct D2D communication), or for determining proximity of a first UE to a second UE. With D2D communication, part of or all communication data may be transmitted directly between two D2D UE without going through a network infrastructure.
In the modes 2 and 3 as shown in FIG. 1, D2D UE may serve as a relay, such that a remote D2D UE at the edge of, or outside, coverage of a cellular network may communicate with the cellular network via the relay UE, and two D2D UE may perform D2D communication with each other via the relay UE.
The inventor/inventors finds/find out that existing D2D technology is disadvantageous at least as follows.
When a condition of D2D UE and/or a relay D2D UE (that serves as a relay of the D2D UE) changes, such as in response to determining that the D2D UE and/or the relay D2D UE moves, the relay UE serving the D2D UE has to be replaced. There is no solution for replacing relay UE in relayed transmission of D2D communication, impacting smooth D2D communication of D2D UE through the relay UE.